Today, people of the world need a lot of energy for daily living.
For obtaining energy to meet human need, it is not necessary to search outside the earth. Each year, solar radiation provided the earth with about 5×1021 BTU of energy. This quantity is about equivalent to 1.4×1018 KWH (kilowatt hour). About 32% of the direct solar radiation is immediately reflected back to space. Most of the absorbed energy is eventually radiated back into space with the characteristic black body temperature of about 10 degree Celsius. A very small fraction of the energy is converted to coal, crude oil and other form and stored for a period of more than 100 years. The short term stored energy available before black body radiation back into space is mostly in the form of ocean surface temperature change (increase, 43% of the energy) and water vapor evaporation (23%). It is estimated that about two percent of the solar radiation energy has later been converted to wind energy. Technology to extract energy from the wind and ocean is at the threshold of able to supply the energy need of the whole world.
The electricity need for the world is about 1.4×1013 KWH a year. The electricity need is about 0.001 percent of the absorbed solar radiation energy. Generated electricity represented the most important form of economically significant human energy need. It is also possible to convert electrical energy into other form to provide human with almost all energy need. Total solar radiation is about 5,000 times the world energy need. Using the estimate that forty percent of the solar radiation energy has been converted to ocean thermal energy, the ability to use 0.03 percent of the ocean thermal energy would be sufficient to provide all the electricity need.
In tropical area, ocean surface temperature is likely to reach 28 degrees Celsius or more. At the depth of 500 meter, the temperature is 10 degrees Celsius. At even greater depth (about 1000 meters), the temperature is 4 degrees Celsius, the maximum water density temperature. The available thermal energy for each square kilometer (to 500 meter depth) is roughly equivalent to the energy of 250 meter of water at a temperature difference of 15 degrees Celsius. The value is
            (      250      )        ⁢          (      100      )        ⁢          (      1000      )        ⁢          (      100      )        ⁢          (      1000      )        ⁢          (      100      )        ⁢          (      15      )        =                    (        3.75        )            ⁢              (                  10          15                )            ⁢              (        calories        )              =                            (          1.57          )                ⁢                  (                      10            16                    )                ⁢                                  ⁢        Joules            =                        (          4.3          )                ⁢                  (                      10            9                    )                ⁢                                  ⁢        Kilo        ⁢                  -                ⁢        Watt        ⁢                  -                ⁢        Hour            
Each square kilometer of ocean may receive a maximum of (1.35)(109) watts of energy. Using 8 hours [(2.88×104) seconds] as the equivalent time during each day that maximum solar input is possible, the possible maximum daily energy input is about (3.9×1013) Joules. The available heat content of the ocean surface (to 500 meter depth) is equivalent to more than 400 days of deposited solar energy. The heat content of the ocean to 1000 meter depth is equivalent to 1000 days of deposited solar energy. For each square kilometer of the tropical ocean, the stored thermal energy is about (3.9×1016) Joules, or 9.2 Megaton of TNT. The energy stored in 10 square kilometer of the tropical ocean is greater than the energy of any hydrogen bomb ever made. The ocean is actually a much better energy reservoir then the oil deposit for the world. The energy is replenished daily by the sun. As long as the earth stay is orbit around the sun and the solar radiation stay at close to the current level, the energy supply is available.
If the OTEC plant is situated close to a significant electricity user, the generated electricity may be connected to the power grid. The energy use efficiency will be higher. If the energy storage form is liquefied hydrogen, the energy use efficiency will be lower as it is unlikely that one hundred percent of the hydrogen energy can be converted back to electricity again.